B. Chen et al., EVOLUTION OF THE TROPOSPHERIC SPLIT JET OVER THE SOUTH-PACIFIC OCEAN DURING THE 1986-89 ENSO CYCLE, Monthly weather review, 124(8), 1996, pp. 1711-1731
A case study investigation into the meridional and horizontal circulat
ion over the South Pacific Ocean is presented for the 1986-89 El Nino-
Southern Oscillation (ENSO) cycle. Using the European Centre for Mediu
m-Range Weather Forecasts (ECMWF) analyses, annual average fields are
created for the years before and after the 1987 minimum (warm phase) a
nd 1989 maximum (cold phase) in the Southern Oscillation index. The an
alyses reveal a shift in the split jet stream over the South Pacific s
ector (180 degrees-120 degrees W) from a strong subtropical jet can) a
nd weak polar front jet (PFJ) during the warm phase to a weak STJ and
strong PFJ during the cold phase. Analysis of the momentum budget reve
als how the split jet in the upper troposphere over South Pacific Ocea
n evolved during the 1986-89 ENSO cycle. During the warm phase: the st
rong STJ is associated with advection of the mean flow momentum Aux fr
om the Australian sector, which is approximately balanced by a large n
egative ageostrophic term; the PFJ is primarily associated with eddy m
omentum convergence, which is partially counterbalanced by the ageostr
ophic term. During the cold phase, the weakened STJ is related to an i
ncreasingly negative ageostrophic term and a less positive mean flow m
omentum convergence. The strengthened PFJ is associated with an increa
se in the convergence of eddy momentum flux that is mainly composed of
2.5-6-day poleward momentum transport from midlatitudes and 7-30-day
equatorward momentum transport from high latitudes. In general, the im
pacts of eddy stress on the STJ and the mean momentum divergence on th
e PFJ in this sector are small. The variations in the split jet may re
flect the poleward propagation of the ENSO signal via the South Pacifi
c convergence zone. The implications for the high southern latitudes a
re discussed as interannual variations are found in the low-level east
erlies near Antarctica and the Amundsen Sea low.